The present invention relates to a mechanical scanning type ultrasonic imaging apparatus and, more particularly, to an ultrasonic imaging apparatus which offers high scanning speed, improved transmission/reception S/N ratio, and improved resolution of a tomographic image.
Mechanical scanning type ultrasonic imaging apparatuses are widely used in the examination of living bodies. An endoscope/ultrasonic diagnostic apparatus combines such an ultrasonic imaging apparatus with an endoscope so as to perform diagnosis of an object to be examined. In such an endoscope/ultrasonic diagnostic apparatus, an ultrasonic transducer is rotatably mounted at the distal end of an endoscope and is driven through a drive shaft by a motor mounted at operation section of the endoscope. The endoscope scanning section has an angle sensor for detecting scanning angle. An ultrasonic wave reflected from an object, i.e., an echo, is converted into an electric echo signal by the ultrasonic transducer. The echo signal from the ultrasonic transducer is amplified by a preamplifier and output to an external lead through a mechanical contact, e.g., a brush. The echo signal at the lead is supplied to a display device and displayed as a tomographic image of the object. In order to increase tomographic image resolution, the frequency of a drive pulse can be increased. However, ultrasonic waves in the object attenuate in proportion to the square of the frequency. Detection sensitivity deteriorates abruptly upon such attenuation. In order to compensate for such sensitivity deterioration, the drive pulse is processed, e.g., pulse-compressed, to increase its level.
In the conventional ultrasonic imaging apparatus described above, a flexible shaft is used as a power transmission member for transmitting power to the transducer. When the transducer is driven at high speed, mechanical resonation occurs due to the mass of the transducer and the elasticity of the flexible shaft. Then, the transducer cannot be driven at a constant speed and irregularities occur during each rotation. Such irregular rotation results in flare or fogging, and thus poor image quality, in the resultant tomographic image. In order to prevent this, conventionally, the transducer is driven at low speed. However, the scanning speed drops at low transducer speeds, and an image of high quality cannot be easily obtained for objects with rapid movements such as a man's heart. In order to prevent the above-mentioned mechanical resonation, the rigidity of the flexible shaft can be increased. However, when a flexible shaft having a relatively high rigidity is used, the diameter of the endoscope insertion section is increased, and flexibility of the endoscope is impaired.
In a conventional ultrasonic imaging apparatus, the echo signal from the transducer is supplied to the display device through the brush as described above. Therefore, when the rotating speed of the transducer is increased, the brush vibrates, chattering increases, and the S/N ratio is lowered.
In order to increase resolution, the transmission output, i.e., the drive pulse, is increased in level by pulse compression. More heat is generated by the transducer because of its low conversion efficiency. The transducer can then be easily degraded and has a short life.